Curling is a deviation of a slab from its original shape, usually caused by either temperature or moisture differentials (or both) within the slab. Curling is a curved shape bending upwards or downwards at the edges. This occurs when top and bottom  of the slab change size (shrink) at different rates. Some causes for curling might be: a significant difference in moisture content or temperature between top and bottom of the slab, concrete slab is relatively thin, distance between joints is large, slab is unreinforced. Below are some ways to reduce the likelihood of such problems:

-Use best possible curing system and consider extending the curing period

-Use lowest practical water slump and avoid retempering with water

-Avoid use of vapor barriers under slab

-Use mix design principles that minimize concrete shrinkage. These consist of using the largest possible aggregate size, minimal cement content, etc among other factors.

-Use smallest contraction joint spacing as possible

-Consider increasing thickness of concrete slab and/or utilize slab reinforcement

-Consider admixtures that reduce shrinkage/curling

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